HVAC system efficiency improvements have provided considerable reductions in energy use. For example, many high efficiency furnaces, air conditioners, and air handlers now have Annual Fuel Utilization Efficiency (AFUE) ratings greater than 90%. However, many blower motors used to move the air in these systems have not seen significant efficiency improvements and have much lower efficiencies. As furnaces and air conditioners have become more efficient, the fraction of total energy consumption attributed to blower motors has increased, thus making blower motors a greater contributor to overall HVAC system energy use.
Blower motor inefficiencies are magnified when a blower motor is operated for extended hours beyond that needed solely for heating and cooling. For example, some users choose to operate their blower motor continuously by setting a fan control switch to the “on” position. This circulation mode of operation reduces temperature stratification, minimizes start drafts from duct work, improves humidity control, and increases the effectiveness of associated air cleaners employed in conjunction with the HVAC system. By selecting the “on” position, the blower motor operates continuously, and the associated thermal feature, (i.e., either heating or cooling) operates on the “demand” setting of the thermostat. When in the “on” position, the blower motor typically operates at the speed used for cooling, even when the thermostat is set to heat mode. This speed is usually well in excess of what is necessary to achieve the air circulation benefits outlined above, causing excess energy usage and noise. In addition, with the blower switch in the “on” position, the system can no longer select a speed for cooling or heating and instead continuously runs at the continuous fan speed. Even when systems are designed to select the proper speed in a multiple speed motor, for example, as disclosed in U.S. Pat. No. 4,815,524, the speed available for blower “on” use is higher than necessary for such operations, and can be responsible for cold spot corrosion, requiring a shut down period disclosed in the '524 patent. The increased operation time also leads to greater energy use.
Many of the above-described inefficiencies result from the type of blower motor used in HVAC systems. HVAC systems traditionally use fixed speed or multiple speed permanent split capacitor (PSC) motors. These motors generally have two or more independent power connections to accommodate two or more heating or cooling modes of operation. The heating or cooling power inputs are normally connected to different winding taps in the PSC motor to provide somewhat different operating speeds for the blower in the respective modes of operation, allowing the OEM or installer to select the operating speed by appropriate connection of the taps to the respective heating and cooling power connections. The energizing of these AC power connections to the motor is controlled by activation of a temperature switch and a relay driven from the thermostat.
An example of a fixed speed PSC motor M used in residential HVAC systems is shown in FIG. 1 and generally identified as 10A. The illustrated motor has two winding taps to accommodate a heating fan speed and a cooling fan speed. The fan speed is controlled by a furnace control board which receives control signals from a thermostat or other control device. Another exemplary PSC motor M is shown in FIG. 2 and generally identified as 10B. This motor has four winding taps to accommodate two heating fan speeds and two cooling speeds. The fan speed is controlled by a furnace control board with a cool/heat relay, a low/high cool relay, and a low/high heat relay. As with the motor shown in FIG. 1, the furnace control board receives control signals from a thermostat or other control device. Other similar HVAC systems may include two heating stages and a single cooling stage or any other combination of heating and cooling speeds. The single phase AC supply voltage (normally 115 VAC or 230 VAC) for both the motors of FIGS. 1 and 2 is supplied by connections L1 and N, where L1 represents the hot side of the AC supply, and N is neutral, which is at earth potential in a typical 115 VAC residential distribution system. (In normal 230 VAC systems, another hot supply line would be substituted for the neutral line N.)
PSC motors such as those shown in FIGS. 1 and 2 are reasonably efficient when operated at high speed, but their efficiencies may drop down into the 20% range when operated at low speeds. Because air conditioner evaporator coils need higher airflow than furnace heat exchangers, the blower motor operates at a lower speed during furnace operation, where it is less efficient, and at an even lower speed still during continuous fan “on” operation, where it is least efficient.
Because of the above-described inefficiencies of PSC motors, many newer HVAC systems use variable speed motors such as brushless permanent magnet (BPM) motors and corresponding electronic variable speed motor controllers. The speed of a BPM can be electronically controlled and set specifically to match the airflow requirements for each application, thus permitting more efficient operation. Also, BPM motors use power approximately proportional to the cube of motor speed, whereas PSC motors use power approximately proportional to motor speed. Therefore, as motor speed drops, BPM motors use less power than PSC motors. This is particularly important when operating the blower continuously for circulation as described above.
While variable speed motors are often superior to PSC motors, replacing an existing PSC motor with a variable speed motor has required costly, time-consuming, and complex changes in the mechanical, wiring, or control configuration of the system. Variable speed motor systems configured for replacement of PSC motors in existing HVAC systems have been developed, but many have relatively complicated control and sensing systems. For example, some systems require the installation of a temperature sensor in the outlet ductwork of the HVAC system for controlling the speed of the motor based upon temperature. Other replacement systems require the connection of low voltage control signals directly from the thermostat to the motor. Making these connections can be cumbersome and difficult in an existing HVAC system. Moreover, these known systems lack the sensitivity to operate blowers at low operating speeds and do not benefit from the relays and control functions in existing furnace control boards.
Still other replacement systems use the control functions of existing furnace control boards but lack standby power when the furnace control board does not call for motor operation. This makes it impossible to program start and stop delays, ramp-down or ramp-up features, or other control features directly into the variable speed motor.
It would therefore be desirable to provide an improved HVAC replacement motor for a PSC motor to realize the advantages of a variable speed blower motor without requiring significant changes to the HVAC system. It would be further advantageous to reduce the complexity of such replacement systems by utilizing simple control circuits and eliminating the need for extensive additional wiring, such as that used in conjunction with traditional variable speed motors and existing replacement variable speed motor systems. It would also be advantageous to provide an HVAC blower motor that could be more easily customized with start and stop delays, motor ramp-up speeds, and/or motor ramp-down speeds.